1. Field of The Invention
This invention relates to a subsystem of an image processing apparatus of the lathe bed scanning type for holding sheets of thermal media on a vacuum imaging drum and more specifically to holding sheet media on a revolving vacuum imaging drum whose surface has been optimized in selected areas to a specified roughness to increase the holding force applied to the media without increasing the vacuum applied to the drum.
2. Background Art
Color-proofing is a procedure that is used by the printing industry for creating representative images of the material to be printed without the high cost and time that is required to actually set up a high-speed, high volume, printing press to produce an example of an intended image which may need to be corrected to satisfy or meet the customers requirements resulting in a large loss of profits.
One such commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. Nos. 5,268,708 and 5,341,159 is for an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media in which dye from a sheet of a dye donor is transferred to the thermal print media, by applying a sufficient amount of thermal energy to the dye donor to form the intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem or write engine (which includes a lathe bed scanning frame, translation drive, translation stage member, print head, and vacuum imaging drum); and a receiver and dye donor exit transports.
The operation of the image processing apparatus comprises metering a length of the thermal print media from a material assembly or carousel. The thermal print media is then measured and cut into sheet form of the required length and transported to the vacuum imaging drum, registered, wrapped around and secured onto the vacuum imaging drum. Next a length of dye donor (in roll form) is also metered out of the material supply assembly or carousel, then measured and cut into sheet form of the required length. It is then transported to and wrapped around the vacuum imaging drum, such that it is superposed in the desired registration with respect to the thermal print media (which has already been secured to the vacuum imaging drum).
After the dye donor is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and dye donor on the spinning vacuum imaging drum while it is rotated past the writing head. The translation drive then traverses the write head and translation stage member axially along the vacuum imaging drum, in coordinated motion with the spinning vacuum imaging drum. These movements combine to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the dye donor is then removed from the vacuum imaging drum. This is done without disturbing the thermal print media that is beneath it. The dye donor is then transported out of the image processing apparatus by the dye donor exit transport. Additional dye donors of a different color are sequentially superimposed with thermal print media on the vacuum imaging drum. These are then imaged onto the thermal print media as previously mentioned, until the intended image is completed. The completed image on the thermal print media is then unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport.
The material supply assembly comprises a carousel assembly mounted for rotation about its horizontal axis on bearings at the upper ends of vertical supports. The carousel comprises a vertical circular plate having six material support spindles. These support spindles are arranged to carry one roll of thermal print media receiver material, and four rolls of dye donor to provide the four primary colors used in the writing process to form the intended image, and one roll as a spare or for a specialty color. Each spindle has a feeder assembly to withdraw the thermal print media from the spindles to be cut into a sheet form. The carousel is then rotated about its axis into the desired position, so that the thermal print media receiver or dye donor (in roll form) can be withdrawn, measured, and cut into sheet form of the required length, and then transported to the vacuum imaging drum.
The write head includes a plurality of lasers diodes which are tied to a print-head and can be individually modulated to supply energy to selected areas of the thermal print media in accordance with an information signal. The write-head of the printer includes one end of a fiber optic array or print head, having a plurality of optical fibers coupled to the diode lasers. The write-head is movable relative to the longitudinal axis of the vacuum imaging drum. The dye is transferred to the thermal print media as the radiation, transferred from the diode laser to dye donor by the optical fibers, is converted to thermal energy in the dye donor.
Existing image processing apparatus designs employ a multi-chamberd vacuum imaging drum for lead-edge control. One appropriately controlled chamber applies vacuum that holds the lead edge of the thermal print media donor sheet material. Another chamber, separately valved, controls vacuum that holds the trail edge of the thermal print media, to the vacuum imaging drum. With this arrangement, loading a sheet of thermal print media receiver and dye donors require that the image processing apparatus feed the lead edge of the thermal print media receiver and dye donors into position just past the vacuum ports controlled by the respective valved chamber. Then vacuum is applied, gripping the lead edge of the a thermal print media receiver and dye donors against the vacuum imaging drum surface.
Unloading the thermal print media receiver and dye donors (to discard the used dye donors or to deliver the finished thermal print media to an output tray) requires the removal of vacuum from these same chambers so that an edge of the thermal print media receiver or dye donors are freed and project out from the surface of the vacuum imaging drum. The image processing apparatus then positions an articulating skive into the path of the free edge to lift the edge further and to feed the thermal print media receiver or dye donors, to a discard receptacle or an output tray.
The receiver and dye donor exit transports consist of a dye donor waste exit and the imaged receiver sheet material exit. The dye donor exit transport comprises a waste dye donor stripper blade disposed adjacent the upper surface of the vacuum imaging drum. In the unload position, the stripper blade is in contact with the waste dye donor on the vacuum imaging drum surface. When not in operation, the stripper blade is moved up and away from the surface of the vacuum imaging drum. A driven waste dye donor transport belt is arranged horizontally to carry the waste dye donors, which is removed by the stripper blade from the surface of the vacuum imaging drum to an exit formed in the exterior of the image processing apparatus. A waste bin for the waste dye donors is separate from the image processing apparatus. The imaged receiver sheet material exit transport comprises a movable receiver sheet material stripper blade that is disposed adjacent to the upper surface of the vacuum imaging drum. In the unload position, the stripper blade is in contact with the imaged thermal print media on the vacuum imaging drum surface. In the inoperative position, it is moved up and away from the surface of the vacuum imaging drum. A driven receiver sheet material transport belt is arranged horizontally to carry the imaged thermal print media removed by the stripper blade from the surface of the vacuum imaging drum. It then delivers the imaged thermal print media with the intended image formed thereon to an exit tray in the exterior of the image processing apparatus.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. Throughput, measured by the number of intended images per hour, is, limited in part, by the vacuum imaging drums rotational speed. With some constraints imposed by the technology itself, the faster the vacuum imaging drum can rotate (without the centrifugal forces lifting or separating the thermal print media receiver and dye donors from each other or off from the vacuum imaging drum causing an image defects or resulting in a media jam and loss of the intended image or catastrophic damage to image processing apparatus), the faster the intended image can be exposed by the image processing apparatus onto the thermal print media the higher the throughput of the image processing apparatus will be increased.
However, with the existing image processing apparatus how the physical characteristics of the thermal print media and the dye donor interface with each other and to the circumferential recess limits the rotational speeds of the vacuum imaging drum that are possible. At high rotational speeds (in excess of 1000 RPM) of the vacuum imaging drum, increased air turbulence and centrifugal force can separate the dye donor from the thermal print media resulting in an image defect in the intended image, or lift the thermal print media receiver and dye donor off the vacuum imaging drum surface which can cause media jams within the image processing apparatus resulting in a loss of the intended image output or, at worse, cause a catastrophic failure to the image processing apparatus.
Known approaches to solving this problem are cumbersome. Adding external clamping components to the vacuum imaging drum would introduce added mechanical complexity to the vacuum imaging drum design and would cause the vacuum imaging drum to be out of round by as much as 80 microns (.mu.m). This would render the image quality of the intended image to be outside the requirements for the process, since the focus tolerance requirement for the write head which the runout tolerance of the vacuum imaging drum is part of is in the ten micron range (10 .mu.m) to meet the image quality requirement. Clamps would also have a clearance problem since the working distance of the print head to the surface of the thermal print media loaded on the vacuum imaging drum is approximately 0.030 inches.
Another way to counteract the force pulling the thermal print media receiver and dye donor from the rotating vacuum imaging drum would be to increase the vacuum level in the interior of the vacuum imaging drum. The vacuum is applied to the thermal print media receiver and dye donor by vacuum holes and grooves in the vacuum imaging drum surface. Achieving a higher vacuum comes with some penalties such as: an increase in the cost of the blower that produces the vacuum, complex vacuum coupling adding mechanical noise to the rotation of the vacuum imaging drum, a higher acoustical output of the image processor, as well as higher customer operating costs (electrical consumption of the product is increased). In addition there is a limit to how high the vacuum level can be without distorting the media, hence decreasing the image quality. While the above approaches increase the RPM limit of the vacuum imaging drum to some extent, they do not over come the problem of the physical characteristics of the thermal print media and the dye donor interface with each other and to the circumferential recess.